Over time the internal quantum efficiency of light-emitting diodes (LEDs) has become very good. For example III-V devices made by metal-organic chemical vapor deposition (MOCVD) typically have internal quantum efficiencies of about 90%. However, the external quantum efficiency is much lower due to poor light transmission from the semiconductor to the air. This is due to several problems, given here by example for AlGaInP devices. Similar issues apply to LEDs, lasers, and other optoelectronic devices made from any semiconductor with a high index of refraction.
The absorbing nature of the most common substrates for AlGaInP requires engineering structures in the film stack to reflect the emitted light away from the substrate. The doped semiconductor layers deposited on top of the light emitting layers are also absorbing. These doped layers are used for improving current spreading and achieving good contact resistance to the metallic contacts on the device. The high index of refraction of the AlGaInP semiconductor (n≈3.3) and the low index of refraction of air (n=1) causes significant reflection of the emitted light if there is no index-matched and non-absorbing optical transition between the semiconductor and the air. Suitable materials for index-matching are difficult to find, and commonly used materials are not matched.